This invention relates to radiometers and, more specifically, to a pyroelectric detector which may be releasably attached to optical sampling apparatus such that the same pyroelectric detector may be alternately used for measuring the power of a continuous wave light source and the energy of a pulsed light source.
The science of radiometry is typically concerned with the accurate measurement of the power and energy of optical radiation (e.g., from lasers and other light sources) in terms of the fundamental units of watts and joules, respectively. Radiometry is distinguished from photometry where light intensity is measured, the word "light" referring to the total integrated range of wavelengths to which the human eye is sensitive.
The ideal radiometer is one that can accurately measure optical radiation regardless of its wavelength, duration or magnitude. Several basic detectors can be employed to usefully measure the radiation over certain regions of operation. Coupled with these detector types are a number of signal processing and detection schemes that may be employed, depending on the temporal characteristics of the input radiation. These temporal properties can generally be classified into two categories; pulsed and continuous radiation. For measuring continuous radiation, power is generally the quantity of interest. When the radiation is pulsed, the total energy (optical power integrated over the duration of the pulse) is normally the quantity of interest. Thus two basic types of radiometers exist, power and energy meters.
A basic energy meter in wide use today utilizes pyroelectric detectors which are used to measure the energy of a pulsed light source. The pyroelectric detector is a thermal detector that outputs a current that is proportional to the rate of change of its temperature. The pulse of light to be measured (energy) is absorbed in the black coating of the detector and is thermally and electrically integrated. The resulting electrical step amplitude is then proportional to the optical energy striking the detector. The instrument digitizes this waveform in real time, measures and computes the amplitude of this pulse, displaying the result in terms of joules. The pyroelectric detector is capable of fast response, broad spectral response, and an insensitivity to DC effects. It is widely used in radiometric systems for industrial temperature measuring systems, the detection of light, and in the analysis of lasers, however; it normally cannot handle high average power levels.
A basic power meter in wide use today utilizes the thermopile detector which is a device composed of a plurality of thermocouples connected in series. A voltage is developed in response to temperature differences between the hot and cold junctions. It may be simply viewed of as a weak battery that converts radiant energy into electrical energy. These devices are relatively slow as compared to pyroelectric detectors (typically 1000 times) but can handle higher average power levels and as a result are commonly used to measure the power of a continuous beam of light. For this purpose a DC voltmeter calibrated in terms of watts is normally employed.
Low duty cycle optical choppers have also been used for many years for beam attenuation purposes; however, this has always been used to extend the range of conventional power measurement techniques which, as aforementioned, are inferior in terms of response times and range as compared to the capabilities of the pyroelectric detector.
It is therefore a principal object of the present invention to provide a radiometer comprising a pyroelectric detector in selective combination with optical sampling apparatus whereby the pyroelectric detector may be used by itself to measure the energy of a pulsed light source, or alternatively be operably attached to the optical sampling apparatus to measure the power of a continuous wave light source.
It is another object to provide a power meter in the form of a pyroelectric detector in combination with optical sampling apparatus which is able to withstand power densities exceeding 2.5 KW/cm.sup.2, and with greater speed of response and greater dynamic range.
It is a further object to provide a power meter in the form of a pyroelectric detector in combination with optical chopping apparatus which is capable of measuring the power of a continuous wave beam in the range of at least 20 micro-watts to 100 watts in substantially less than a second.
Other objects will in part be obvious and in part appear hereinafter.